Possible Cause 1
A hum or other excessive noise can be caused by a misaligned or bent motor shaft. Likewise, this noise can be created if there is a minor fault in the transmission dividing the motor from the driven equipment, or even in the equipment itself. First feel the motor shaft for any obvious faults or bends. If nothing is noticeable, disconnect the motor shaft from the transmission and driven equipment, and then turn it on. If the noise is no longer present, then either the transmission or the driven equipment is at fault, not the motor.
Possible Cause 2
If the noise is still present, there may be either an electric imbalance or a mechanical imbalance. Turn the motor on and let it run for a few moments. Then cut the power. If the motor stops the instant the power is cut, then the problem is electrical rather than mechanical. An electrical imbalance is caused primarily by a worn or unwound part inside the motor which results in an uneven electromagnetic field, causing the rotor to wobble and vibrate as it spins, resulting in a hum. To repair this, remove the casing from the motor and check to see if there is any charring or discoloration inside. Look at the coils of wire within to make sure that they are tightly wound. If nothing of the sort can be seen, then one may need to take the motor to a professional for a more thorough analysis.
Possible Cause 3
If, when the power is disengaged from the running engine, the noise continues until the motor runs to a stop, the problem is a mechanical imbalance. Such imbalances may include a bent motor shaft, damaged rotor, bad bearings or debris caught within the motor casing. Remove the motor casing and look within to see if any obvious signs of wear or damage can be found. Individual parts, if broken or bent, can be replaced, however if the integrity of the motor's frame has been damaged, it may be preferable to obtain a new motor altogether.